1. Field of the Invention
The invention relates to the field of shoes in general, and more specifically to the application of microspheres in shoe material formulations to yield desired shoe properties. Even more specifically, the invention relates to a shoe component prepared from a closed cell foam incorporating microspheres to yield controlled cell size, cell size distribution and expansion characteristics. In another embodiment, the invention also relates to an inner sole for a shoe prepared from a reactive polymer component and a curing agent where the curing agent is incorporated in microspheres to yield a material conformable to the user's foot shape.
2. Description of Related Art
Shoe components often must exhibit a combination of key performance variables by virtue of the stresses resulting from long-term shoe wear. These variables include strength characteristics such as impact strength and durability, as well as characteristics more closely associated with shoe comfort, such as softness, resilience and conformability to the user's foot shape.
Foamed compositions based on ethylene vinyl acetate (EVA) or polyurethane have been used extensively in shoe component manufacture. Typically, such materials, when used in shoe sole applications, should exhibit a balance of density and hardness and compression set. Although the hardness of a foam is primarily a function of the hardness of the base polymer matrix, the cell structure may also play a role in terms of a foams resistance to penetration. For instance, foams with a large population of voids near the surface may have less resistance to penetration than one with less surface voids. Foams may have primarily open or closed cell structures and a distribution of cell sizes. Foams used in shoe applications preferably have a primarily closed cell structure as this leads to reduced compression set, so that the shoe part, such as shoe sole, will not flatten with prolonged compressive loading. The closed cell foam will also impart good resilience and elasticity. Finally, the closed cell structure will lead to a shoe part with greater endurance and durability while under continuous load.
It has been known that the density and cell size of, and the amount of closed cells in, a foamed shoe material can be adjusted by using different types and/or amounts of blowing agents in the foam blowing process. During the shoe molding process, thermal energy is applied to the composition, which chemically activates the foaming agent, releasing a gas such as carbon dioxide (CO2) or nitrogen (N2), which causes a volumetric expansion in the resin matrix resulting in voids. Typically such reaction processes are controlled by the usual process variables of time, temperature, pressure, type and concentration of the various reagents. However, it is still difficult to control the precise nature of the foams cellular structure such as the degree to which the foam is open celled or closed celled.
Similarly, in addition to durability, a shoe must be comfortable to wear, an important factor in which, is how the shoe inner sole conforms to the shape of the wearers foot. This conformability feature is especially important in sports shoes, which can be subjected to either rigorous motion, as in athletic shoes, or long walks over undulating terrain, as in a golf shoe. Shoes that are conformable to the individual wearer can be obtained using several different methods. Professionally crafted shoe inserts are one expensive and time-consuming option. Another option includes custom fitting the shoe by measuring the shape of the foot in three dimensions, and forming the shoe inner sole using the measurement data. However, this process requires a specially designed device to measure the shape of the foot in three dimensions, a separate measurement for each wearer's foot, and a custom production of the shoe insert or inner sole of the shoe using the measurement data. Overall, it is difficult to economically incorporate this process into a mass production line for a shoe.
Numerous other processes are available that provide a shoe offering a more custom fit. Examples of these processes include the use of stock shoe inlays for heel and/or arch support, inserts composed of a gel contained within membrane layers, and air bladders into which air is pumped to achieve support under the foot. In addition, one type of an insert used in sport boots or shoes is heated in a microwave and then inserted and immediately worn in the boot or shoe. The insert shapes itself to the wearer's foot as the insert cools. The aforementioned processes require special apparatus and manipulation, and/or simply do not provide adequate customization for the wearer of the shoe.
U.S. Pat. No. 6,280,815 to Ersfeld et al. (“the Ersfeld patent”) also discloses a shoe insert that custom forms to, and permanently retains, the shape of a foot that it will support and the shoe in which it will be used. The Ersfeld patent discloses a conformable substrate layer and an outer layer. At least a portion of the conformable substrate layer is impregnated with a storage-stable, setable resin that sets after less than 10 minutes of exposure to an activator, preferably water. While this process requires only the use of water, preferably warm, it is inconvenient to have to wet the insert, wear the shoe and damp insert for a set amount of time while maintaining the foot in a fixed position, and remove the insert to allow it to completely dry. Also, because the position of the foot is held constant, this process produces an insert that does not comfortably accommodate a range of foot activity
Accordingly, there is a need for a shoe component material which includes a closed cell foam having controlled size, expansion characteristics, and distribution in the resin matrix. There is also a need for a custom-fit shoe or insert that can be prepared in an easy and cost-effective manner, and that conforms precisely to the wearers shoe shape, such that the resulting shoe can comfortably accommodate a range of foot activity. There is also a need for a product which custom forms to, and permanently retains, the shape of the foot of an individual user after the shoe has been manufactured and purchased. The present invention satisfies all of these needs.